Dosage-dispensing devices for material in powder form are used in particular to dispense small quantities of a substance into small target containers with a high degree of precision. In many cases, the target containers are placed on a balance in order to weigh the dosage material delivered by the dosage-dispensing device, so that it can subsequently be further processed according to a given purpose.
The dosage material to be dispensed is contained for example in a dosage-dispensing unit which includes a source container and a dispensing head. It is desirable to dispense the dosage material through a narrow discharge orifice of the dosage-dispensing unit, so that it can be filled in a targeted stream even into a container with a small opening diameter.
Ideally, dosage material in powder form flows, propelled by gravity, in a uniform stream out of a small discharge orifice, comparable to the sand flowing in an hourglass. In this ideal case, the flow of the dosage material could be controlled simply by varying the aperture cross-section of the discharge orifice with a suitable shutter device. Unfortunately, most of the materials used in practice don't behave according to the ideal case. Many of the particles that make up the dosage material are not conforming to an ideal flow behavior. It is often observed that particles clot together during the dispensing process, with the consequence that the dosage-dispensing unit becomes clogged up. Only by mechanical intervention can the clogging be removed, so that the dispensing process can be continued.
In addition, many powders are compressible, leading to the formation of clumps of compacted powder which can clog the discharge orifice partially or completely. A typical example is cornstarch, which exhibits a strong tendency of the powder particles to stick together. In addition, cornstarch is very compressible. In view of these properties, cornstarch is often used as a sample material to test the performance of dosage-dispensing devices. The problems which are caused by the sticking-together and the compressibility of a dosage material, also referred to as “stickiness” of a dosage material, have led to numerous proposals for a solution and for designs of dosage-dispensing devices, specifically their dispensing heads. There are dispensing heads, for example, which have conveyor screws designed to move the sticky dosage material from the source container to the discharge orifice of the dispensing head. The state of the art further includes stirring and scraping devices which scrape the dosage material from the interior walls of the source container and propel it towards the dispensing orifice. The stirring devices also have the purpose to prevent the formation of so-called bridges of the dosage material in the source container. Instead of, or in combination with, a stirring device, there are also tapping and vibrating devices among the known state-of-the-art solutions.
A possible way to solve the aforementioned problems is disclosed in US published application 2006/0011653 A1. A dosage-dispensing device designed to dispense dosage material in powder form in quantities from a few milligrams to a few grams with a weight tolerance of less than ±5% of the target weight includes a dosage-dispensing unit which is in essence composed of a source container and a dispensing head. From the part that connects to the source container, the inside wall of the dispensing head is conically tapered towards a discharge orifice. In the operating state, the discharge orifice is at the bottom of the dosage-dispensing unit and thus below the source container. Propelled by gravity, free-flowing dosage material inside the dosage-dispensing unit flows towards the discharge orifice. The latter is equipped with a shutter valve which serves to regulate the outflow of dosage material. The dosage-dispensing device includes means for vibrating and/or tapping the source container and can further include a stirrer device capable of rotary motion about, as well as linear motion along, its axis of rotation.
In tests with cornstarch, severe difficulties were encountered in the attempts to dispense the material out of differently designed dosage-dispensing units. The free-flow ability of this kind of dosage material depends directly on how loosely it is aggregated, i.e. by its bulk density. In some tests it was found that the dosage material was compacted, instead of loosened up, by the use of a stirrer device and/or a vibrating means. This had the result that, even with the discharge orifice fully opened, no dosage material could be moved out of the dosage-dispensing unit.
A solution for this problem is disclosed in EP 1959244 A1. The dosage-dispensing device disclosed there includes a holder device and at least one receiving device. The receiving device is designed so that at least one dosage-dispensing unit can be removably seated in it. The dosage-dispensing device further includes at least one actuator to agitate the receiving device. The receiving device is pivotally supported on the holder device, so that the actuator can impart oscillating swivel movements to the receiving device. The actuator performs this function by directing a short sequence of several tapping strikes at the receiving device, causing the latter to swing like a pendulum. The pendulum frequency of the receiving device is preferably out of step with the impact frequency of the actuator. This has the effect that the shock waves or mechanical pulses generated by the strikes are propagating in different directions. This counteracts the tendency of the dosage material to clump together. Nevertheless, there may still be some clumping of the dosage material in isolated instances. The known state of the art includes moving components, for example stirrers or scrapers, which are arranged in the dosage-dispensing unit and serve to counteract the clotting of the dosage material. These movable components require a complex mechanism and are prone to wear out. In addition, the additional movable parts can cause damage in particular to delicate dosage materials.
It is therefore an objective to provide a dosage-dispensing device which has the capability to condition the dosage material in the dosage-dispensing unit prior to the dispensing process and for the dispensing process, specifically to loosen up the dosage material and to thereby counteract the clogging of the dosage-dispensing unit, but wherein no movable parts are arranged in the interior of the dosage-dispensing unit.